Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CIJ).
The first technology, “drop-on-demand” ink jet printing, provides ink drops that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TIJ).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink may be perturbed in a manner such that the liquid jet breaks up into drops of ink in a predictable manner. Printing occurs through the selective deflecting and catching of undesired ink drops. Various approaches for selectively deflecting drops have been developed including the use of electrostatic deflection, air deflection and thermal deflection mechanisms.
In a first electrostatic deflection based CIJ approach, the liquid jet stream is perturbed in some fashion causing it to break up into uniformly sized drops at a nominally constant distance, the break-off length, from the nozzle. A charging electrode structure is positioned at the nominally constant break-off location so as to induce an input image data dependent amount of electrical charge on the drop at the moment of break-off. The charged drops are then directed through a fixed electrostatic field region causing each droplet to deflect by an amount dependent upon its charge to mass ratio. The charge levels established at the break-off point cause drops to travel to a specific location on a recording media or to a gutter, commonly called a catcher, for collection and recirculation. This approach is disclosed by R. Sweet in U.S. Pat. No. 3,596,275 issued Jul. 27, 1971, Sweet '275 hereinafter. The CIJ apparatus disclosed by Sweet '275 consisted of a single jet, i.e. a single drop generation liquid chamber and a single nozzle structure. A disclosure of a multi jet CIJ printhead version utilizing this approach has also been made by Sweet et al. in U.S. Pat. No. 3,373,437 issued Mar. 12, 1968, Sweet '437 hereinafter. Sweet '437 discloses a CIJ printhead having a common drop generator chamber that communicates with a row (linear array) of drop emitting nozzles each with its own charging electrode. This approach requires that each nozzle have its own charging electrode, with each of the individual electrodes being supplied with an electric waveform that depends on the image data to be printed.
One known problem with these conventional CIJ printers is variation in the charge on the drops caused by the image data-dependent electrostatic fields from adjacent electrodes associated with neighboring jets. These input image data dependent variations are referred as electrostatic crosstalk. Such electrostatic crosstalk can produce visible artifacts in the printed image. Katerberg disclosed a method to reduce or eliminate the visible artifacts produced by the electrostatic crosstalk interactions by providing guard gutter drops between adjacent print drops across the jet array in U.S. Pat. No. 4,613,871. However, the presence of electrostatic crosstalk from neighboring electrodes limits the minimum spacing between adjacent electrodes and therefore resolution of the printed image.
Thus, the requirement for individually addressable charge electrodes in traditional electrostatic CIJ printers places limits on the fundamental nozzle spacing and therefore on the resolution of the printing system. A number of alternative methods have been disclosed to overcome the limitation on nozzle spacing by use of an array of individually addressable nozzles in a nozzle array and one or more common charge electrodes at constant potentials. One method uses control of the jet breakoff length as disclosed by Vago et al. in U.S. Pat. No. 6,273,559 issued Aug. 14, 2001, Vago '559 hereinafter. Vago '559 discloses a binary CIJ technique in which electrically conducting ink is pressurized and discharged through a calibrated nozzle and the liquid ink jets formed are stimulated to breakoff at two distinct breakoff distances which differ by less than the wavelength λ of the jet defined as the distance between successive ink drops or ink nodes in the liquid jet. Two sets of closely spaced electrodes with different applied DC electric potentials are positioned just downstream of the nozzle adjacent to the two breakoff locations and provide distinct charge levels to the relatively short breakoff length drops and the relatively long breakoff length drops as they are formed. This results in differential deflection between drops having the two distinct breakoff lengths when placed in a uniform electric field region. Limiting the breakoff length locations difference to less than λ restricts the stimulation amplitudes difference that must be used to a small amount. For a printhead that has only a single jet, it is quite easy to adjust the position of the electrodes, the voltages on the charging electrodes, and print and non-print stimulation amplitudes to produce the desired separation of print and non-print droplets. However, in a printhead having an array of nozzles part tolerances can make this quite difficult. The need to have a high electric field gradient in the droplet breakoff region also causes the drop selection system to be sensitive to slight variations in charging electrode flatness, electrode thicknesses, and component spacings that can all produce variations in the electric field strength and the electric field gradient at the droplet breakoff region for the different liquid jets in the array. In addition, the droplet generator and the associated stimulation devices may not be perfectly uniform down the nozzle array, and may require different stimulation amplitudes from nozzle to nozzle to produce particular breakoff lengths. These problems are compounded by ink properties that drift over time, and thermal expansion that can cause the charging electrodes to shift and warp with temperature. In such systems extra control complexity is required to adjust the print and non-print stimulation amplitudes from nozzle to nozzle to ensure the desired separation of print and non-print droplets.
B. Barbet and P. Henon also disclose utilizing breakoff length variation to control printing in U.S. Pat. No. 7,192,121 issued Mar. 20, 2007 (Barbet '121 hereinafter). Barbet '121 addresses some of the issues by increasing the difference in the breakoff lengths between print and non-print drops. T. Yamada disclosed a method of printing using a charge electrode at constant potential based on drop volume in U.S. Pat. No. 4,068,241. B. Barbet in U.S. Pat. No. 7,712,879 disclosed an electrostatic charging and deflection mechanism based on breakoff length and drop size using common charge electrodes at constant potentials.
These drop control systems use a charging electrode that is held at a fixed electrical potential relative to the jets in conjunction with image data dependent breakoff lengths. As they employ a charging electrode that is common to the array of nozzles, print drops are not affected by electrostatic crosstalk due to the image dependent voltage on charging electrodes associated with neighboring drops. These drop control systems however do produce print drops that are charged, albeit at a magnitude that is below that of the catch drops. The print drop charge can result in electrostatic interactions between neighboring or nearby print drops which cause alterations of drop trajectories and result in drop placement errors and degraded print quality on the recording media. As the packing density of nozzles in a print head increases to provide higher print resolution, the electrostatic interactions between neighboring or nearby print drops increase causing larger alterations in drop trajectories.
As such, there is an ongoing need to provide a high print resolution continuous inkjet printing system that prints with selected drops from an array of nozzles without the print defects of these drop control systems.